According to the website “Gender Specific Colors,” it would seem that assigning color to gender is mostly a 20th century trait. It would also seem that at one time, the color associations were reversed when color first came into use as a gender identifier… “At one point pink was considered more of a boy’s color, (as a watered-down red, which is a fierce color) and blue was more for girls. The associate of pink with bold, dramatic red clearly affected its use for boys.

“There has been a great diversity of opinion on the subject, but the generally accepted rule is pink for the boy and blue for the girl. The reason is that pink being a more decided and stronger color is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl.” [Ladies Home Journal, June, 1918]http://histclo.hispeed.com/gender/color.html – “Gender Specific Colors”

In the United States: “The current pink for girls and blue for boys wasn’t uniform until the 1950’s.

It would also seem that Nazi Germany had something to do with the association of pink with femininity:

“Catholic traditions in Germany and neighboring countries reverse the current color coding, because of the strong association of blue with the Virgin Mary…the NAZIs in their concentration camps use a pink triangle to identify homosexuals. (The yellow star of David is the best known symbol, used of course to identify Jews. The German system was quite complicated, using various symbols an colors to identify criminals, political prisinors, an a whole range of other groups). The NAZI’s choice of pink suggests that it by the 1930s was a color that in Germany had become associate with girls.” – “Gender Specific Colors” http://www.answerbag.com/q_view/706366

”‘The NAZI’s choice of pink suggests that it by the 1930s was a color that in Germany had become associate with girls.’”

This isn’t true actually, from what I’ve heard: pink was considered a masculine colour back then (as it was similar to red) – the triangle badges identifying homosexuals in concentration camps were pink because they liked other men, not because of any perceived femininity.